Use display: block for code blocks inside <pre>

Fixes some minor formatting issues that can come up with code blocks,
since they were previously being treated as inline-block. For example,
this caused list items containing a code block to have a mis-positioned
list item marker.

tildes/scss/_base.scss

@@ -180,6 +180,7 @@ pre {
   overflow: auto;
 
   code {
+    display: block;
     line-height: 1.1rem;
   }
 }